A study of reliability of elementary facts in the developmental histories of pediatric patients in a clinic population is reported. The results show that mothers do not report gestation time reliably, that many mothers are incorrect when they state that forceps were not used in the delivery, that few mothers can report accurately the immediate difficulties at the delivery of their infant. Feeding histories revealed many discrepancies in duration of nursing and knowledge of formula composition. Many mothers forget or overlook a significant number of illnesses. On the other hand, the mother's evaluation of difficulty of labor and delivery agrees with that of the physician; facts concerning weight at birth, and at subsequent yearly intervals, and details of motor development are reported with accuracy. The significance of reliability is stressed in interpreting the relevance of factual developmental data to differential diagnosis in deviant behavior, perinatal studies, etc., and further areas of investigation of reliability are suggested.